Ehrlichia chaffeensis antibodies are proteins made by the immune system in response to infection with Ehrlichia chaffeensis, a bacterium transmitted by the lone star tick. This infection, known as human monocytic ehrlichiosis (HME), affects white blood cells and can cause fever, chills, headache, muscle aches, and, in severe cases, life-threatening complications.
Two main classes of antibodies are measured: IgM and IgG. IgM antibodies are usually the first to appear, often within the first week or two of illness, and suggest a recent or active infection. IgG antibodies develop more slowly, usually by the second to fourth week, and indicate either an ongoing or past infection. Rising IgG levels in paired blood samples taken several weeks apart provide the strongest evidence of active disease.
Because antibody production takes time, early blood tests may be negative even if infection is present. For this reason, doctors may repeat testing or use additional methods like PCR (a test that detects the bacteria’s DNA) to confirm the diagnosis. Antibody tests can also remain positive for months after recovery, reflecting past exposure rather than active illness.
Untreated ehrlichiosis can progress quickly, so doctors often start antibiotics such as doxycycline based on clinical suspicion, even before test results return. Early treatment greatly improves outcomes, making timely recognition and testing for Ehrlichia antibodies an important part of care when symptoms follow a tick bite.
